This invention relates generally to electronic differential pressure transmitters, and more particularly to a transmitter of the force-beam type which operates on an open loop principal.
A differential pressure transmitter is used to measure various process variables such as flow rate, liquid level and density when these process variables are convertible to a differential pressure signal. One important application of the force-balance principle is in a differential-pressure transmitter. Such transmitters are widely used in industrial process systems to produce an output signal suitable for transmission to a remote station for operating indicator and/or process control equipment.
In a transmitter of this type, an elongated force bar is pivoted about a transverse axis and an input force derived from a differential pressure capsule and corresponding to the flow rate of the fluid to be measured is applied to the force bar to produce a torque about its fulcrum. Also applied is a rebalance torque which tends to hold the bar motionless.
The rebalance torque is developed by a negative feedback loop that includes a detector to sense any slight change in force bar position due to an unbalance of torques. The detector generates a corresponding feedback signal that is directed to a feedback motor. The motor, in turn, applies to the force bar a force in opposition to the input force. This feedback signal is maintained proportional to the differential pressure being measured and is usable to produce an output signal for transmission to a remote control station or to an indicating or recording device.
In a force-balance instrument of the electrical type, such as that disclosed in U.S. Pat. No. 3,832,618, the feedback system is provided with an electric motor and the output signal is electrical in nature, whereas in the pneumatic type, such as that disclosed in U.S. Pat. No. 3,742,969, the motor is in the form of a pneumatically-actuated bellows and the output signal which is applied to the bellows is fluidic in nature.
In a force-balance transmitter, there is virtually no movement of the force bar over the full-scale range of operation. This virtual absence of movement is highly advantageous, for it effectively eliminates non-linearity and other errors of the type encountered in so-called motion-balance instruments.
In a force-balance instrument, it is important to be able to change the operating range or span of the instrument. In an instrument of the type disclosed in U.S. Pat. No. 3,564,923, in which the feedback system is electrical in nature and employs a feedback motor, the force-balance mechanism includes a vernier range-changing structure in the form of a flexured reaction member and a rotatable support element adapted to change range without altering the static balance of the instrument.
A differential-pressure force-balance transmitter of the type heretofore known acts on a closed-loop principle in that the input force applied to the force bar is balanced by a feedback force applied thereto through a feedback loop.
Also known are electronic differential pressure transmitters of the so-called open-loop type. In one form of an open-loop differential transmitter, the low and high pressure fluid inputs are applied to a pair of process diaphragms which are hydraulically coupled to opposite sides of an electrode diaphragm. The deflection of the electrode diaphragm depends, therefore, on the difference between the low and high input pressures. This deflection is converted into a corresponding change in the capacitance established between the electrode diaphragm and the meter body. The change in capacitance is detected by a capacitance bridge and amplified to generate an output signal which represents differential pressure.
The advantage of an open-loop differential pressure transmitter of the above-described transducer type over a standard, closed-loop transmitter of the force-balance type is that the former is not only relatively light-weight and less expensive to manufacture, but it is also substantially insensitive to mechanical vibration. On the other hand, the force-balance type is characterized by high accuracy, a wide zero suppression range and the absence of electrical connections in the meter body, an important safety factor.